1. Field of the Invention
The present invention generally relates to a method of manufacturing an ultrasonic sensor.
2. Description of the Related Art
An ultrasonic sensor is configured to generate an ultrasonic wave and receive a part or the entire generated ultrasonic wave reflected back to the ultrasonic sensor. The ultrasonic sensor is used in, for example, an obstacle detecting system that detects an object by intermittently generating an ultrasonic wave in a particular direction(s) and receiving a part or the entire generated ultrasonic wave reflected from an obstacle situated in this direction. The ultrasonic sensor may also be used for a range finding system configured to measure a distance from a target object. The range finding system, which uses the ultrasonic wave sensor to transmit an ultrasonic wave and receive a reflected ultrasonic wave echoing in response to the transmitted ultrasonic wave, measures the difference in time between the ultrasonic wave being transmitted and the reflected ultrasonic wave being received and calculates the distance from a target object according to the measured time difference.
FIG. 11 shows an exemplary configuration of a conventional ultrasonic sensor 10.
The ultrasonic sensor 10 mainly includes a sensor case 11, a piezoelectric element 12 provided on a bottom part of the sensor case 11 and, using the bottom part of the sensor case 11 as a vibration plate, a terminal (conductive material) 13 in conductive connection with both electrodes situated on the upper and lower side of the piezoelectric element 12 while being connected to an external circuit of the sensor case 11, and a wire 14 connected to the terminal 13 and the piezoelectric element 12. The sensor case 11 has a configuration allowing a filling material 15 to be supplied therein for preventing liquid droplets from entering (See Japanese Laid-Open Patent Application Nos. 9-284896, 11-266498, 2000-32594, and 2002-209294).
Typically, in order to provide a piezoelectric property during a stage of manufacturing a piezoelectric element of an ultrasonic sensor that generates ultrasonic waves, a polarizing (poling) process (process of applying a strong direct current electric field) is performed on the piezoelectric element. By performing the poling process, electric dipoles in the piezoelectric element (in this example, a piezoelectric ceramic) become aligned in a certain direction and the dipole moment remains even after removal of the electric field due to the ferroelectric property of the piezoelectric ceramic. Thereby, the piezoelectric ceramic attains a piezoelectric property. However, the poled piezoelectric ceramic has a characteristic of creating a voltage and causing polarization loss in a case where thermal change occurs in the poled piezoelectric ceramic (e.g., by application of heat).
In a case where the filling material 15 serving to protect the sensor case 11 is supplied into the sensor case 11 during a stage of manufacturing the piezoelectric sensor 10, heat is applied to the piezoelectric element. Thereby, polarization loss occurs in the piezoelectric ceramic.